DER Format to create key.der binary format of the private key.
CombinedPEM Format to create tls-combined.pem containing tls.key + tls.crt.
Added Unit and e2e tests for secret with Additional output format.
Feature flag AdditionalCertificateOutputFormats to enable feature.
Signed-off-by: Thierry Sallé <seuf76@gmail.com>